Glioblastoma multiforme (GBM), or “glioblastoma,” is a high-grade astrocytoma representing the most common form of primary brain tumors. The successful treatment of patients with GBM is still a major challenge, and the median survival rate is 14.5 months after diagnosis (Stupp et al., Promising survival for patients with newly diagnosed glioblastoma treated with concomitant radiation plus temozolomide followed by adjuvant temozolomide. J Clin Oncol. 2002 Mar. 1; 20(5):1375-82; Stupp et al., Radiotherapy plus concomitant and adjuvant temozolomide for glioblastoma. N Engl J Med. 2005 Mar. 10; 352(10):987-96).
Most anti-cancer therapeutics have defined targets such as oncogenes, enzymes or DNA, which may be localized to distinct intra-cellular compartments like the nucleus, mitochondria or cytosol. However, determining which targets and targeting strategies to utilize in order to provide the most effective clinical treatment for cancers such as GBM remains a major challenge.
Such targets may include the Eph receptors. (Pasquale (2010) Eph receptors and ephrins in cancer: bidirectional signalling and beyond. Nature Reviews Cancer 10:165-180). Ephrin-A ligands generally bind promiscuously EphA receptors; however, EphA3 is specifically activated by ephrin-A5 (eA5). EphA3 activation regulates several physiological processes like cell adhesion, migration and cellular morphologic responses that have also been related to tumor growth, invasiveness and metastasis (Beauchamp and Debinski (2012) Ephs and Ephrins in Cancer: Ephrin-A1 Signaling Semin Cell Dev Biol. 23:109-115). EphA3 is overexpressed in GBM, in particular on tumor-initiating cells (Day et al., EphA3 maintains tumorigenicity and is a therapeutic target in glioblastoma. Cancer Cell 2013 23: 238-428).
EA5 binds with high specificity also to the Eph receptor A2, inducing its down-regulation and degradation (Ferluga et al. (2012) Biological and structural characterization of glycosylation on ephrin-A1, a preferred ligand for EphA2 receptor tyrosine kinase J Biol Chem; 288:18448-57). EphA2 is overexpressed in glioblastoma (GBM) tumor specimens when compared to normal brain and can be specifically targeted by an ephrin-A1 (eA1)-based cytotoxin, killing GBM cells expressing the receptor (Wykosky et al. (2005) EphA2 as a novel molecular marker and target in glioblastoma. Mol Cancer Res; 3:541-551; Wykosky et al. (2007) A novel, potent, and specific ephrinA1-based cytotoxin against EphA2 receptor expressing tumor cells. Mol Cancer Ther 6:3208-3218). EphA2 has been shown to be involved in GBM invasiveness (Miao et al. (2014) EphA2 promotes infiltrative invasion of glioma stem cells in vivo through cross-talk with Akt and regulates stem cell properties. doi: 10.1038/onc.2013.590. [Epub ahead of print]).
Studies have also shown that eA1 is able to activate EphA2 in a monomeric form (Wykosky et al. (2008) Soluble monomeric EphrinA1 is released from tumor cells and is a functional ligand for the EphA2 receptor. Oncogene; 27:7260-7273). It can also be mutagenized to improve binding affinity (Lema Tome et al. (2012) Structural and functional characterization of the monomeric EphrinA1 binding site to the EphA2 receptor. J Biol Chem; 287:14012-22), and the glycosylation of the ligand promotes activation of the receptor, stabilizing the formation of heterotetramers on the cell membrane (Ferluga et al. (2012) Biological and structural characterization of glycosylation on ephrin-A1, a preferred ligand for EphA2 receptor tyrosine kinase. J Biol Chem; 288:18448-57).
EphB2 is also over-expressed in GBM cells, especially invasive ones, but not in normal brain (Nakada et al. (2010) The phosphorylation of ephrin-B2 ligand promotes glioma cell migration and invasion. 126(5):1155-65).
Another target in cancers such as glioblastoma is the IL-13 receptor IL-13Rα2, which is expressed in >75% of GBM tumor specimens (Debinski et al. (1999) Receptor for interleukin 13 is a marker and therapeutic target for human high-grade gliomas. Clin Cancer Res. 5(5):985-90; Saikali et al. (2007) Expression of nine tumour antigens in a series of human glioblastoma multiforme: interest of EGFRvIII, IL-13Ralpha2, gp100 and TRP-2 for immunotherapy. J. Neurooncol. 81(2):139-48) and is characterized as a cancer/testes like antigen (Debinski et al. (2000) Molecular expression analysis of restrictive receptor for interleukin 13, a brain tumor-associated cancer/testis antigen. Mol Med. 2000 May; 6(5):440-9). IL-13Rα2 is believed to act as a decoy receptor (Bernard et al. (2001) Expression of interleukin 13 receptor in glioma and renal cell carcinoma: IL13Ralpha2 as a decoy receptor for IL13 1. Lab Invest 81(9):1223-31). It has been shown that the IL-13 ligand binds to the IL13Rα2 receptor and is internalized through receptor mediated endocytosis (Kawakami et al. (2001) The interleukin-13 receptor alpha2 chain: an essential component for binding and internalization but not for interleukin-13-induced signal transduction through the STAT6 pathway. Blood 97(9):2673-9; Debinski et al. (1995) Human glioma cells overexpress receptors for interleukin 13 and are extremely sensitive to a novel chimeric protein composed of interleukin 13 and pseudomonas exotoxin. Clin. Cancer Res. 1(11):1253-8). Thus, drugs attached to the IL-13 ligand can be internalized and delivered specifically inside the glioma cells. However, some glioblastoma cells are resistant to targeting of IL-13Rα2 (Nguyen et al. (2011) IL-13Rα2-Targeted Therapy Escapees: Biologic and Therapeutic Implications. Transl Oncol. 4(6):390-400).